Please help! My son has a bump on the back of his head

My son is 10 week old,and I have noticed he has a small squishy (as fluid filled) bump on the back of his head.We went to the doctor she's not sure what it is.she said it may happened during delivery (27 hours labor) and no need to worry.just keep an eye on it.but im worry to death everytime I hold him and feel his bump on my hand. Do you have any idea what causes this? Should I go to another doctor and get it checked?